This invention relates to a system for encoding composite color television signals into interframe encoded signals for transmission thereof and/or decoding the interframe encoded signals for reception of the color television signals.
For digital transmission of television signals in general, an interframe code transmission system is already known as an effective band-compression transmission system for reducing as far as possible the number of bits to be transmitted and for thereby raising the efficiency of transmission channels. The interframe encoding makes use of interframe correlation of the television signals and is capable of effectively compressing the frequency band of television signals for slowly changing objects.
The present-day color television signals, such as those according to NTSC, PAL, and SECAM systems, are composite color television signals wherein frequency multiplication is carried out for a luminance signal, such as a Y signal, and a carrier chrominance signal. The latter signal results from modulation of a subcarrier by chrominance signals, such as an I and a Q signal. When applied to a composite color television signal, a sophisticated interframe code transmission system is objectionable because prediction errors between frames for the chrominance signals grow large to render it impossible to carry out the effective band compression. More particularly, let the NTSC system be taken into consideration. The subcarrier frequency f.sub.SC is 455/2 times the horizontal scanning frequency f.sub.H, which is, in turn, 525 times the frame frequency f.sub.F. Namely, the subcarrier frequency f.sub.SC is an odd integral multiple of a half (1/2) of the frame frequency f.sub.F. The subcarrier therefore has a phase difference of 180.degree. between two adjacent frames. This inevitably amplifies by a factor of about two the interframe or frame-to-frame difference signals for the chrominance signals. As a result, the effective band compression is not achieved. In order to solve the problem arising from the phase difference of the subcarrier, the interframe encoding has been carried out by resorting to the so-called separate coding where the carrier chrominance signal is preliminarily demodulated on the transmitter side into separate baseband signals, such as the I and Q signals, and wherein the luminance and chrominance signals are subsequently separately subjected to interframe encoding. On the reception side, the original NTSC signal is reproduced from the luminance signal and the interframe decoded chrominance signals. For detail, reference may be had, if desired, to an article contributed by Kazumoto Iinuma et al. to IEEE Transactions on Communications, Vol. COM-23, No. 12 (December 1975), pp. 1461-1466, and entitled "Interframe Coding for 4-MHz Color Television Signals."
The separate coding, however, is defective in that the system therefor is complicated. For example, the transmitter has to comprise a decoder for the NTSC signal and an analog-to-digital converter for each of the luminance and chrominance signals. The receiver has to comprise a digital-to-analog converter again for each of the luminance and chrominance signals and an encoder for the NTSC signal. Furthermore, the separate coding is defective in that the wave form of the signals are inevitably distorted during the demodulation for transmission and the re-modulation for the reception otherwise to degrade the quality of the pictures.